Simple question. My mum's been over this week and I got her a PAYG. She
still has plenty of credit left, and is going back to Spain... no idea when
she'll be back, but it probably won't be within a year.

Rather than take her card back and use her credt, I told her she may as
well hang on to it for her next visit. But if she doesn't use it, will it
expire??

I wouldn't have thought so, but when you buy eg one of those £5 phone
cards, the credit expires if you don't use it within a period of time
(which I think should be illegal). I'd rather she hang on to the Oyster as
I have a season ticket, but I wouldn't want the money to go to waste if in
a year or more of non-usage the credit will disappear....